Xerographic device



April 5, 1966 R. w` GuNDLAcH 3,244,083

XEROGRAPHIG DEVICE Filed Oct. 23, 1962 4 SheetS--Sheet 1 INVENTOR ROBERTW. GUN DLACH April 5, 1966 R. w. GuNDLAcH XEROGRAPHIC DEVICE 4Sheets--Sheet 2 Filed Oct. 25, 1962 HIGH VOLTAGE.

POWER SUPPLY 'N25 INVENTOR LE M ROBERT W. GUNDLACH fmf QOQQ April 5,1966 R. w. GuNDLAcH XEROGRAPI'IIC DEVICE 4 SheetsSheet 5 Filed Oct). 23,1962 INVENTOR. ROBERT W. GUN DLAC H April 5, 1966 R. w` GuNDLAcl-l3344,0

xERoGRAPHIc DEvIcE Filed Oct. 23, 1962 4 Sheets-Sheet 4 HIGH VOLTAGEPOWER SUPPLY -25 n'.. ...n n."

H'GH voLTAGE powi-:R suPPLY -26 w o (L -74 '23 i 22a n' zfi'. u'.. 'uz',nn v[+++ INVENTOR. ROBERT W. GUNDLCH TTORNEY United States Patent YorkFned oct. 23, 1962, ser. No. .232,494 6 Claims. (Cl. SDS-1.7)

This invention relates to xerogr'aphy and, in particular, .to Va novelmethod and apparatus for separating a toner .image from an insulatingsurface while preserving the electrostatic charge pattern on which it isdeveloped.

Xerography i's a Well-known method of image recording in which a latentelectrostatic charge pattern formed on a sufiicientl'y non-conductivelmaterial is visualized by an appropriate development process. Thedeveloped image is usually transferred to any suitable image transfermaterial such as paper, especially when a reusable plate is employed inthe forrnation of the electrostatic charge pattern.

The latent electrostatic charge pattern may be formed in various ways.One well-known technique involves as a first step deposition of auniform electrical charge on a xerographic plate comprising aphotoconductive layer overlying a conductive hacking. This first step iscarried out in the dark, or 'in light to which the photoconductive layeris insensitive, by means of a corona discharge device such as thatdisclosed in Walku-p U.S. Patent 2,777,957. Exposure of the chargedplate to an optical image selectively dissipates the charge inlight-struck areas, thus producing an electrostatic charge lpatternconforming to the optical image.

One of several known development techniques may be employed to renderthe electrostatic charge pattern visible' For instance, the chargepattern may be dusted `with an oppositely charged pigmented resinpowder,

termed toner. As already indicated, it may be preferred that the tonerimage be transferred to image transfer material thus freeing thexerographic plate for further use. Electros'tatic transfer techniqueshave been successfully employed for this purpose. Accordingly, an imagetransfer web, such as a sheet of paper or plastic, may be laid over thedeveloped image and an electrical charge applied to the upper surface asby means of a corona discharge device already mentioned. The chargedeposited on the web establishes an electrostatic field which draws thecharged toner particles from the plates and attaches them to thetransfer Web. The transfer web may then be separa'ted from the platesuch as by peeling.

, Duringseparation, electrostatic field in the air gap between thetransfer web and plate remains essentially constant so that theelectrical potential rises as the spacing increases. The lines of forcewhich hold the toner particles in place are accordingly stretched. Whenthe spacing has so increased, that 'the electrical potential reaches acritical point with respect to the atmospheric conditions, air breakdownor sparking occurs in the air gap. This phenomenon results in distortionof the electrostatic charge pattern and in diminution of electricalContrast. In conven'tional xerography, in which only one transfer ismade from a developed image, this Spark- 'ing is of no consequence.However, it must `be avoided if the electrostatic charge pattern is tobe preserved for subsequent development and transfer Steps.

'One method of electrostatic image preservation is disclosed in ByrneU.S. Patent 2,951,443, and comprises separating the developed image fromthe insulating surface while applying a uni'form and constant electricalpotential to the image transfer web by means of a conduc'tive membersuch as a roller. However, it isV frequen'tly difficult `to determineexacly what potential should be applied to the conductive member toaccom- .plish neutralization of the transfer charge. The transfer chargemay vary because of power supply fluctuations, spacing between coronadischarge transfer device and imageweb, etc., and sparking may not beprevented if the applied potential is insufficient to neutralize thetransfer charge or if the transfer charge is over-neutralized- Theinstant invention is directed to preventing air breakdown or sparking byinsuring that a neutralizing charge of correct magnitude is applied tothe transfer web. This is accomplished by means of a second vcoronadischarge device operated in' conjunction with the transfer coronadischarge device. By Operating the two devices electrically connected inaccordance with this invention, transfer charge neutralization may beeifected so that Sparkfree transfer can be conveniently attained.

Accordingly, it is a principal object of this invention to provide novelmeans to preserve an electrostatic charge pattern in its originalcondition without distortion during the separation step in theXerographic process.

Ot'her and further objects of this invention are made apparent by thefollowing description in which:

FIGURE 1 illustrates an electrostatic charge pattern on a non-conductiveplate;

FIGURE 2 illustrates development of the plate carrying an electrostaticcharge pattern;

FIGURE 3 illustrates transfer-separation according to the presentinvention;

FIGURE 4 illustrates a corona discharge device suitable for use in thepresent invention;

`FIGUR'E 5 shows a preferred power supply for use in the presentinvention;

'FIGURE 6 is a diagrammatic view of a continuous machine for productionof multiple copies according to the present invention;

`FIG. 7 illustrates an embodiment wherein the transferlseparation coronadevices are connected by a variable resistor; and

FIG. .8 illustrates an embodiment wherein the shields of the coronadevices comprise an integral structure.

Illustrated in FIGURE 1 is an electrostatic charge pattern 11 on thesurface of insulating plate 12. The electrostatic charge pattern 11 isdesignated by plus marks and, for purposes of this description, is'assumed to be of positive polarity. However, it is to be understoodthat the charge pattern may be of either positive or negative polarityand may consist of gradations of potential or charge or may be, asvillustrated, areas of charge interspaced by areas of substantially nocharge. The particular charge pattern illustrated in FIGURE 1 may beregarded as a line copy pattern consisting of representative areas ofcharge and absence of charge which desirably can be printed in areas ofblack on a white surface. Insulating plate 12. must be at leastsuficiently non-conductive to support an electrostatic charge during theinstant process. The conventional xerographic plate is suitable for thispur'pose but it is not suggested that this invention is limited thereto.

In FIGURE 2 there is illustrated development of electrostatic` chargepattern 11 by the te'chnique generally known in the art 'as cascadedevelopment." In cascade development, which is deseribed in Walkup U.S.Patent 2,6l8,55'1, a mixture of particles 21 is presented to the surfacebearng the charge pattern. The particles are generally frictionallyelectrostatically charged and the electrostatic fields of force whichexist between the charges on the particles and the charges on thesurface of the plate cause partcle deposition in conformity with theelectrostatic charge pattern resulting in toner image 22.

The developer generally used in cascade development comprises a drymixture of fine toner particles electrostatically adhering to largerparticles generally referred to the finer toner particles as thedeveloper mixture passes" over and in contact with the electric field ofinfiuence of the electrostatic charge pattern. During development,developer is passed across the plate surface either once or a 'number oftimes and the toner particles deposit on the charge pattern to form avisible toner image.

It is to be realized that other known xerographic development techniquesmay be used. Accordingly, it is intended to include within the scope ofthis invention separation of toner images produced by other knownxerographic developing techniques including, but not limited too, loopdevelopment, powder cloud development, brush development, magneticdevelopment, and the like. Specifically, it is-noted that where a rigidplate member is being used, cascade development is generally preferredto produce line copy images and powder cloud development is generallypreferred to produce continuous tone images.

Transfer-separation comprising the present invention is illustrated inFIGURE 3. Particles comprising toner image 22 are shown adhering toinsulating plate 12 in conformity with electrostatic charge pattern 11indicated by the plus marks. Transfer web 23, which is shown in lightcontact with insulating plate 12, may comprise any suitable materialsuch as paper, plastic, or the like. As translfer corona device 24 ismoved rela-tive to insulating plate 12 in the direction shown by thearrow, its positive ccrona discharge overcomes the attractive forces ofelectrostatic charge pattern 11, thus causing toner image 22 to betransferred onto transfer web 23. Transferred `toner image 22a is shownadhering to those portions of transfer web 23 which have been subjectedto the corona discharge from transfer corona device 24. 'Separationcorona device 2'5 emits corona discharge opposite in polarity to that oftransfer corona device 24 and is also moved relative to transfer web 23and insulating plate 12 in |the direction indicated by the arrow.Separation corona device 25 causes neutralization of the charge producedby transfer corona device 24 permitting the separation of transfer web23 from insulating plate 12 without the occurrence of air breakdown orsparking. As a result electrostatic charge pattern 11 remains intactafter the separation step permitting its reuse in the developmentprocess for the formaton of additional toner images. Separation coronadevice 25 need not necessarily be positioned immediately adjacent totransfer corona device 24, as appearing in FIGURE 3. However, the coronadevices would generally be closely positioned to minimize the physicalspace occupied by these elements.

FIGURE 4 shows one of the general types of corona discharge devicessuitable for operation in conjunction with the present invention. It isreferred to as a corotron and is described in greater detail inVyverberg U.S. Patent 2,836,725. The corotron assembly, designated gen-'erally by reference Character 30, comprises two insulating terminalblocks 31 positioned at either end of the assembly and joined togetherby a conductive shield 32. `'Corona wire 33 is a fine conductive strand'mounted on the insulating terminal blocks and runs lengthwise of theassembly from one block to the other beneath shield 32. Corona wire 33forms the corona discharge electrode and may be conductively connectedto a high voltage power source.

It is to be realized that when separation of the transfer web from theinsulating plate surface takes place without transfer chargeneutralization, such as that provided by separation corona device 25shown in FIGURE 3, the capacitance between the transfer web and thesurface of the insulating plate decreases as the gap between the twoincreases. As capacitance decreases, since the charge on each surface isnot varied, the potential increases at a rapid rate during separation.Usually a point is reached at which the potential becomes sufficientlygreat to bring about an electric air breakdown between -thesurfacesyresulting in distortion of the image pattern on the insulatingplate surface. Therefore, separation corona device 25,' by eflfectingthe neutralization of transfer charge, maintains the field-spacerelationship below critical stress conditions. Little or no positivecharge remains on the transfer web 23 but the transferred toner image22a is held on the web by physical forces and the small remainingpositive transfer charge. Sparking or other electric air breakdownphenomena are prevented and the electrostatic charge pattern ismaintained on the insulating plate surface in its original condition.

As already indicated, the neutralization charge must be of correctmagnitude in order to be effective. If the ionic emission fromseparation corona device 25 is either insuflicient or too great,conditions conducive to air breakdown will persist. Of course,neutralization of the transfer charge could be accomplished by using twoseparately powered corona discharge devices for transfer andseparation.V These devices would have to be carefully adjusted so thatthe neutralization charge would be of equal magnitude, but oppositepolarity, to that of the transfer charge. As a practical matter,variations in Operating conditions mentioned earlier make such criticaladjustments extremely difficult i-f not impossible.

Apparatus according to the present invention is inherentlyself-regulating so that exactly the same amount of charge is depositedon the transfer web by each corona device. By forcing the oppositelycharged emission from separation corona device 25 to be automaticallyequal to the emission from transfer corona device 24, completeneutralization of the transfer web is elfected. The present inventionachieves this result, variations in Operating conditionsnotwithstanding.

Reference is again made to 'FIGURE 3 to show how the neutralizationcharge is efi'ectively regulated in accordancc with the presentinvention. Transfer corona device 24 and separation corona device 25 areboth electrically connected to oppositely poled terminals of powersupply 26, which is described in greater detail in connection with'FIGURE 5. For purposes of this description, the corona Wire of transfercorona device 24 is shown connected to the positive side of power supply26y and the corona wire of separation corona device 25 is shownconnected to the negative side although this relation can be reversed.The power supply 26 is electrically insulated or isolate-d from groundas are each of the corona discharge devices. The respective shields ofthe two corona devices are electrically connected by means of wire 27although it is to be understood that this electrical connection may beoptionally attained through a fixed or variable resistor 67, asillustrated in FIG. 7, or by operating the corona devices with theirshields in direct contact, or by forming both such shields as anintegral structure 74, as schematically illustrated in FIG. 8. The othercomponents shown in FIGS. 7 and 8 are the same as those of FIG. 3 andVare designated by the same reference characters. If a resistor were usedVfor this electrical connection, it might, if desired, be use-d tocontrollably re' duce corona emission from the two corona devices.

Since, except for the corona emission contact, the elements mentionedare electrically isolated from their en' vironment, it may be seen thatthe amount of current leaving one corona device must be equal andopposite to that entering the other corona device. With the elementsconnected as shown in FIGURE 3, voltages will automatically adjust toprovide equal emission from the two corona devices. Accordingly, theamount of charge deposited by the corona devices must be equal,resulting in complete neutralization of charge on the transfer web. Thenecessary electrical isolation in the apparatus can be attained bycareful attention to conventional principles of good engineering. Thecorona devices should be supported on high quality insulators andconnected to the power supply by well insulated conductors. The powersupply may be of the conventional transformer rectifier type, but shouldincorporate a well insulated transfor'r 'free of corona. It isespecially advantageous to use the type of transformer in which the highvoltage secondary winding is physically sep'arated from the core andprimary winding. Good insulating should be used `throughout the powersupply and rectifier filament transformers, if used, should have wellinsulated secondary windings. Filter condensers, if employed, should beconnected across the 'high voltage transformer rather than to ground,unless they are of exceptionally low leakage. If a power supply meetingthis description is not available, power supply 26 may alternativelycomprise a convenftional power supply moun'ted on good insulators andpowered by a well insulated high voltage isolaton transformer 'which maybe of the 1:2 type. Various other means for providing high voltage powersources isolated from ground are known and may be employed. Also, powersupply V26 may Vbe replaced by several individual power suppliesprovided each is suitably insulated.

'FIGURE 5 depicts the essential elements of a preferred high Vvoltagepower 'supply for use in this invention. A primary winding 34 is woundon a toroidal iron core 35. High voltage 'secondary coil 36 isinterlinked with the core but is physically separate from the core andprimaryiwinding. A'set of rectifiers 37 transform the high voltagealternating current to direct current on' the order of 10,000 to v15,000volts. A filter condenser'38 may be connected across the rectifieroutput terminal. The entire high voltage 'as'sembly is connected to highvoltage feed through bushings, 39 which may be theA sole mechanicalsupport there'for. Thus, complete electrical isolation exists betweenthe high voltage circuit and `all low voltage elements such as themechanical structure of the power supply, the primary winding, or thelike.

As illustrated in FIGURE 6, the present invention may be suitablyapplied to a continuously Operating machine designed to produce aplurality of copies from a single electrostatic charge pattern. Axerographic drum 40 comprising a photo-conductive insulating layer 41overlying a conductive 'hacking v42 'is suitably mounted to rotate inthe direction indica'ted 'by the arrow. The drum is disposed and adaptedto pass successively through a charging station 50 and exposure station51, a first development station 52, a first transfer-separation station53, a second development station 52a, and a second transferseparationstation 53a and a Cleaning station 54.

The charging, exposing, developing, and Cleaning stations areconventional in the art of xerography. Thus, for example, the chargingstation may be of the type described in connection with FIGURE 4comprising a corona Wire 55 operably connected to power supply 56. Theexposure station 51 may include a projection lens 57 or other means ofimposing a pattern of light and shadow to be recorded on the surface ofxerographic drum 40. First developing station 52 and second developingstation 52a may comprise suitable means or methods for applying anelectroscopic powder or material to the xerographic drum and may, forexample, comprise a fur brush.

In accordance with the present invention, transfer-separation station 53comprises a pair of corotrons electrically connected to the same powersupply 61 in the manner described in connection with FIGURE 3. Transfercorotron 58 is positioned and disposed to eifect the electrostatictransfer of the electroscopically adhering toner image from the surfaceof xerographic drum 40 to the surface of transfer web 59. Separationcorotron 60 is .positioned and disposed to deposit an electrostaticcharge to neutralize that deposited by transfer corotron 58. Theelectrostatic yfields of force at the area of separation are therebymaintained below air breakdown potential so that separation of thetransfer web from the surface of the xerographic drum may be effectedwithout destruction of the electrostatic charge pattern. Theelectrostatic'. charge pattern is again visualized in this embodiment atsecond development station 52a after which transfer ibodied to allowcontinuous cycling while the charging means and the exposure means aremaintaned in an inoperative position V'to -allow production of 'apr'edetermined number of copies.

Although this invention has been described with reference to 'thespecific structure disclosed in this application, it is not intendedthat this invention be limited by lthose specific embodiments. Rather itis intended that the clairns apply -broadly within the spirit a'nd scopeof this invention.

What is cla-imed is:

11. Self-regulating apparatus to successively deposit on a chargeablesurface of an im'age transfer web a `first electrostatic charge and 'asecond 'electrostatic lcharge of equal |magnitude and opposite polarity.to said first electrostatic charge, comprising in combination:

(a) 'a first corona discharge device compris'ing corona Wire positionedwithin a conductive shield;

(b) a second corona discharge device comprising corona Wire positionedwithin a condu-ctive shield; and,

(c) an electrically isolated lhigh voltage power supply; said coronawires of sa-id corona discharge devices being respectively connected tooppositely poled terminals of said power supply, said conduct-ives'hields being electrically connected, said combination of said coronadischarge devices 'and said power supply 'being electrically isolatedfrom its environment, and said corona d'ischarge devices being disposedso that succe-ss-ive areas of said web `are brought into operativeproximity with said first corona discharge device and said second coronad'ischarge device respectively.

2. Appar'atus according to claim 1 wherein said con- |ductive shie'lds:are electrically connected by means of a variable resistor tocontrollably reduce corona emission Vof said coronadischarge devices.

3. Apparatus according to claim 1 wherein said conductive shields ofsaid corona discharge devices comprise an integral structure.

4. Apparatus to produce a plurality of duplicate copies from an originalcomprising:

(a) a xerographic plate in the form of a drum comprising ap'hotoconductive layer overly-ing a grounded conductive b'acking;

('b) means to form an electrostatic 'charge pattern on said xerographicplate conforming .to said original;

(c) developing means to form a visible toner image comprisingelectrically cha'rged powder;

(d) guide means to bring a surface of an image transfer web into contactwith said toner image;

(e) self-regulating transfer-separation means comprising a firstshielded corona electrode to deposit an electrostatic charge on thenon-contacting surface of said web to effect transfer of said powder tothe contact-ing side of said web and a second s'hielded corona electrode.to deposit a neutralizing charge on the non-contacting surface of saidweb, said corona electrodes being 'connected to oppositely poledterminals of a high voltage power supply, the respective shiel-ds ofsaid electrodes being electrically connected, and saidtnansfer-se'paration means being electrically isola'ted from itsenvironment; and,

(f) means to move successive p'ortions `of said plate first intooperative relation with said electrostatic charge pattern forming meansand then into operatro-static charge, said apparatus including a powersuptive relation with a plurality of combinations comply, a firstshielded corona discharge electrode and a secprising in sequence saiddevelop-ing means, said guide ond shielded corona discharge electrode,the improvement means and said transfer-sep-aration means. comprising incombination: I

5. Apparatus to producea plurality of duplicate copie .a -first coronadischarge elect'rode p-artly surrounded 'from an original comprising:'by an electrically conductive Shield;

(a) a xerograp'hic plate in the form of a drum coma second coronadischarge electro-de p-artly surrounded pris-ing a photoconductive layeroverly'ing .an electrically grounded conductive hacking;

by an electrically conductive Shield; a power supply including atransformer having a high (b) means to form an electrostatic chargepattern on 10 voltage secondary winding physically separate from saidxerographic plate conf-orming to said original; the primary winding;

(c) developing means to form a visible toner image means to connect saidelectrodes to opposite poles of comprising electrically charged powder;and, said power supply; and,

(d) electf'icany solated Self-regulatng FaHSfeT-Separa- 'means toelectrically connect the conductive shields tion means comprising afirst shielded corona elecparfly Surmunding said electrodes. trode todeposit on the surface of .an image receiving web opposite to thtsurface of said web brought References Cite by the Examiner into contactwith said toner image to effect trans er of said toner image -to thecontacting surface of said UNITED STATES PATENTS web, a second shieldedcorona electrode to deposit 2,8l2,709 11/1957 Gundlach 101-426 .aneutralizing charge on the non-contacting surface 2,836,725 5/ 1958Vyverberg ZSO-49.5 of said we'b, and an electrically isolated 'highvoltage 2,868,989 1/1959 Haacke 250-495 power supply, said coronaelectrodes being connected 2,895,847 7/1959 Mayo 118 637 X to opposite1yp'oled terminals of said high voltage 2,951,443 9/1960 Byrne 95. 1 7 Xpower supply and respective s'h-ields of said elec- 3,015504 1/1952Caflson et 31 95 1 7 X trodes being electrically connected. 3,124,4573/1964 Schwertz 95 1 7 X '6. In xerographic apparatus for successivelyapplying -to a cha'rgeable surface a first electrostatic charge of onepolar'ity and a second electrostatic charge of equal magnitude and ofopposite polarity to that of said first elec- EVON C. BLUNK, PrimaryExaminer.

WILLIAM D. MARTIN, Examner.

1. SELF-REGULATING APPARATUS TO SUCCESSIVELY DEPOSIT ON A CHARGEABLESURFACE OF AN IMAGE TRANSFER WEB A FIRST ELECTROSTATIC CHARGE AND ASECOND ELECTROSTATIC CHARGE OF EQUAL MAGNITUDE AND OPPOSITE POLARITY TOSAID FIRST ELECTROSTATIC CHARGE, COMPRISING IN COMBINATION: (A) A FIRSTCORONA DISCHARGE DEVICE COMPRISING CORONA WIRE POSITIONED WITHIN ACONDUCTIVE SHIELD; (B) A SECOND CORONA DISCHARGE DEVICE COMPRISINGCORONA WIRE POSITIONED WITHIN A CONDUCTIVE SHIELD; AND, (C) ANELECTRICALLY ISOLATED HIGH VOLTAGE POWER SUPPLY; SAID CORONA WIRES OFSAID CORONA DISCHARGE DEVICES BEING RESPECTIVELY CONNECTED TO OPPOSITELYPOLED TERMINALS OF SAID POWER SUPPLY, SAID CONDUCITVE SHIELDS, BEINGELECTRICALLY CONNECTED, SAID COMBINATION OF SAID CORONA DISCHARGEDEVICES AND SAID POWER SUPPLY BEING ELECTRICALLY ISOLATED FROM ITSENVIRONMENT, AND SAID CORONA DISCHARGE DEVICES BEING DISPOSED SO THATSUCCESSIVE AREAS OF SAID WEB ARE BROUGHT INTO OPERATIVE PROXIMITY WITHSAID FIRST CORONA DISCHARGE DEVICE AND SAID SECOND CORONA DISCHARGEDEVICE RESPECTIVELY.